The goal of this Core is to design, synthesize and evaluate selected chemical modifications to small interfering RNAs (siRNAs) for improved chemical stability and delivery to cells within the cervicovaginal mucosa that are important in HIV transmission. Previous experience with in vivo applications of oligonucleotides (including antigene, antisense, decoy, and ribozyme therapies) has clearly demonstrated the need for chemical modification to improve pharmacokinetics in mammalian systems. Although there are virtually no technical limitations regarding chemical modification of the nucleobase, sugar and phosphate moieties of RNA, interactions of the siRNA with the proteins involved in gene silencing puts constraints on the types of modifications and the number of residues that can be modified without severely impairing the efficacy of the siRNA. Optimization of delivery, stability, and activity of siRNAs for vaginal delivery will require an assessment of a range of chemical modifications for incorporation within and near the termini of siRNAs in combination with cholesterol or CCR5-ligand conjugation for enhancement of uptake into cells susceptible to infection. Finally, the selected siRNAs will be appropriately formulated for clinical intravaginal application. This Core will provide modified and formulated siRNAs to each of the Projects for in vitro and in vivo assays to test the potential of specific modifications for gene silencing and for protection against viral transmission in cell lines, primary cells and cervicovaginal explants (Project by Lieberman), in mice (Project by Palliser) and in rhesus macaques (Project by Veazey). The ultimate commercial product from this study will be a drug that can be used vaginally to prevent infection by HIV. In particular, our aims are to (1) improve biological stability of siRNA duplexes by adding phosphorothioate linkages to the backbone and by modification of the 2'position of the sugar residues;(2) improve cell targeting and permeation by conjugation of cholesterol, CCR5-specific ligands, or mannose to siRNA;and (3) develop an siRNA formulation suitable for clinical intravaginal application.